1. Field of the Invention
This invention relates to a curable composition comprising a solvent solution of a mixture comprising:                (i) at least one hydroxy-functional acrylic polymer; and        (ii) optionally, for especially high solid applications, at least one low molecular weight polyol reactive diluent;        (iii) at least one polyisocyanate;        (iv) a metal catalyst, such as a tin compound, for accelerating the isocyanate/hydroxyl reaction; and        (v) propionic acid.        
The curable compositions of this invention are especially useful as coatings and may typically be utilized as primers, topcoats or as clearcoats and/or basecoats in clearcoat/basecoat compositions and are especially useful in spray applications. In particular, the combination of these materials provides fast reacting, durable coatings having extended pot-life and excellent cure. The compositions of this invention could also be utilized as adhesives, elastomers and plastics. Although it is not our intention to be bound by theory, it appears that the use of propionic acid, due to its ease of handling and evaporation rate, has special utility in spray applications and ambient air cures. Surprisingly, propionic acid has shown superior performance, especially in high solid spray applications, compared to other similar organic acids such as acetic acid.
2. Description of the Prior Art
Two-component curable mixtures comprising polyisocyanates and active hydrogen-containing compounds, such as polyols or polyamines, are well-known in the art to provide excellent performance and cure at low temperatures. However, due to the reactivity of the isocyanates and the active hydrogen-containing compounds, it is often difficult to obtain long pot-lifes of the mixture of polyisocyanate and active hydrogen-containing material and yet still enjoy the benefits of rapid cure. This is especially true for low VOC materials, which will incorporate relatively low levels of solvent
A number of approaches to extending the pot-life of mixtures of polyisocyanates and active hydrogen-containing compounds, have been taught in the prior art. For example, U.S. Pat. No. 3,808,162 teaches polyurethane catalyst combinations of metals with organic acids, including propionic acid in certain polyol/isocyanate applications. U.S. Pat. No. 4,426,510 teaches the use of beta-dicarbonyl compounds, alpha-hydroxy ketones, fused aromatic beta-diketones and beta-hydroxy nitrogen-heterocyclic fused aromatics as pot-life extenders for certain metal catalyzed polyol/poly-isocyanate combinations. U.S. Pat. No. 4,235,766 teaches the addition of certain 2-methyl-2-alkanols to extend the pot-life of isocyanate/polyol mixtures. U.S. Pat. No. 4,096,128 teaches the use of certain triazoles to extend the pot-life of isocyanate/diamine mixtures. U.S. Pat. No. 4,019,933 teaches use of reaction products of tri-functional aziridinyl phosphine oxides with carboxylic acids to extend pot-life. U.S. Pat. No. 4,184,031 teaches the use of a composition of matter comprising a metallo organic compound and a carboxylic acid, or a compound convertible thereto, such as a carboxylic anhydride, by reaction with an active hydrogen compound. U.S. Pat. No. 4,341,689 teaches polyurethanes catalyzed by amines and metal compounds in the presence of pot-life extending amounts of acetic or formic acid, but expressly teaches that propionic acid is not useful due to its slow vaporization. U.S. Pat. No. 4,877,829 teaches the use of organic and inorganic acids, such as benzoyl chloride, p-toluene sulfonic acid, formic acid, acetic acid, benzoic acid, phosphoric acid, hydrochloric acid and the like. U.S. Pat. No. 5,157,100 teaches the use of polyanhydride materials to extend the pot-life of active hydrogen containing materials such as polyols and polyamines with polyisocyanates. The prior art has not, however, taught the solvent borne combination of:                (i) at least one hydroxy functional acrylic polymer;        (ii) at least one low molecular weight polyol diluent;        (iii) at least one polyisocyanate;        (iv) a metal catalyst such as a tin compound; and        (v) a pot-life extending amount of propionic acidto provide low temperature curing, extended pot-life materials having excellent durability, hardness and performance upon cure. This invention also relates to the process of spray application of the solvent borne curable composition and its subsequent cure by exposure to heat, or preferably for certain applications, at ambient temperatures.        